As a trial of Khmer Rouge leaders moves forward, a journalist and filmmaker who spent years with regime leaders warns of the elusive nature of truth.

Speaking on an extra “Hello VOA” Tuesday, Thet Sambath, who conducted extensive interviews over more than 10 years with Nuon Chea and other Khmer Rouge cadre, said the court is facing a huge mandate of truth while only pursuing a case against four top leaders.

“There were not just a few Khmer Rouge leaders involved in starvation, torture and killing,” he said. “There were many people linked to these issues. If you want to learn the truth, you have to call [to the court] many more directly involved people and investigate deeply.”

So far the Khmer Rouge tribunal has tried one man, Duch, the chief of Tuol Sleng prison. It is set to try four more: Nuon Chea, Khieu Samphan, Ieng Sary and Ieng Thirith.

But the process has been slow and has filled many Cambodians with anticipation and, some argue, very high expectations.

“It’s hard to research the truth,” Thet Sambath said Tuesday. “If we have a shallow investigation, we see that this man is wrong, or this unit is wrong. But if we investigate deeply, we find the men that are really involved in the issues; they appear. That’s why with the court, I don’t say, ‘Oh, they are doing a good job because they are capable.’ But I think investigating the Khmer Rouge regime, in my understanding, is very difficult. So they can’t find the truth at all."

“If we investigate who is deeply involved in the issues, receiving the orders, making plans, and we take them all into account, after that we see the truth, what caused the problems,” he said. “If there are only four or five leaders stating the issues, I think it’s not enough.”

In his award-winning documentary, “Enemies of the People,” Thet Sambath faces Nuon Chea and asks why some people were killed. The former leader says they were against the revolution and had to be dealt with.

Such decisions by leaders, and whether they constituted grave atrocities crimes, are at the heart of the upcoming trial.

Five more possible indictments in two further cases remain in question, with tribunal observers concerned that political pressure from the government is curtailing deeper investigation.

Huy Vannak, a spokesman for the tribunal, said one of the court’s main purposes is to seek justice for victims and help them understand how the killing fields came about.

“Through the court, people can see the deeds that occurred in the past, and it is also a process to seek peace and national reconciliation in Cambodia,” he said.

However, Thet Sambath said the work of the Khmer Rouge was by nature secretive, making any search for the truth difficult. That secretiveness was also a leading factor in the killings, he said.

“First, Pol Pot and Nuon Chea didn’t know there were groups against them, and that’s why Pol Pot gave authority to the regions to decide everything,” he said. “That’s the fault of Pol Pot. When Pol Pot knew there were groups opposed to him and spoiling his policies, he responded by arresting them and killing them. That’s why we had the killing fields.”

Pol Pot died in 1998, and the leaders now facing trial are aging. Their trial could take years, giving rise to concerns the defendants may perish before judgment is passed on their actions. For many ordinary Cambodians, justice for the former leaders has come too slowly.

“Lawyers for the accused and the victims have not focused on the details yet,” Chey Theara, deputy director of the Association of Khmer Rouge Victims, said Monday, on the opening day of a preliminary hearing. “So some victims have been kind of disappointed.”